Fertilization typically triggers the dramatic exocytosis of membrane-bound cortical granules from the peripheral egg cytoplasm into the extracellular space. An egg surface of mosaic composition is constructed, derived from membranes of the cortical granules and the original egg plasmalemma. The structure, role and fate of these mosaic surface components has not been fully ascertained. The traditional concept has been that the cortical granule membrane components become permanently integrated into the egg plasma membrane; the egg accomodates the excess membrane introduced into its surface by microvillar elongation. However, identification of coated vesicles in the cytoplasm of the activated egg and recent preliminary studies showing uptake of extracellular surface labels during granule exocytosis suggest that reorganization of the mosaic surface appears to include internalization of membrane through formation of endocytotic vesicles. Using routine TEM and SEM, freeze fracture replication, TEM cytochemistry, and quantitative radiolabel assay, and the large, translucent egg of a freshwater fish as a model system for analysis, this study will: 1) provide further morphological and experimental evidence for the recovery of membrane from the activated (fertilized) egg surface; 2) reconstruct the sequence of endocytosis and determine the intracellular pathway(s) in the egg cortex by which endocytosed membrane is handled; and 3) determine if the mechanism underlying the endocytosis is associated with a subplasmalemmal filamentous system in the egg cortical cytoplasm. The results of this study will provide a more comprehensive understanding of the egg activation process in vertebrates, including humans (where a cortical granule response is known to occur), and insight into the possible role of the early egg surface in the control of later development events. A knowledge of the reorganizational changes that take place over the entire egg surface upon activation has distinct implications for fertility control and will establish an important framework upon which future studies can be conducted on the early program of egg development.